At present, variable speed or adjustable speed electric drives are available that comprise an inverter and an electric motor. However, the speed range of a drive of this type, over which the drive can provide constant power, tends to be quite limited. Efforts to increase this speed range have resulted in systems which are heavy, costly, and inefficient.
Some hydrostatic drives, which consist of a hydraulic pump and a hydraulic motor, have been used for the above purpose. Use of electro-mechanical devices, however, to achieve constant power over a greater speed range, has generally not been successful. For example, difficulty has been encountered in sustaining high holding torques at speeds which are close to zero. In an alternative approach, which uses a range box driven by an electric drive, it has proved difficult to achieve seamless range shifts.